Electrical amplifier circuit



Feb. 17, 1931. FH DRAKE'ET AL 1,792,970

ELECTRICAL AMPLIFIER C IRCUIT Filed May 4, 1928 2 Sheecs--ShLe-ei'l l Y Feb. 17, 1931.

F. H. BRAKE ET A l. 1,792,970 ELECTRICAL AMPLIFIER CIRCUIT 4 Fled May 4, 1928 2 SheetS-Sheeb 2 //afaflce /as/b-a v9 me h//e 7; l L 5 Patented Feb., 17, 1931 UNTTED STATES PATENT osi-ica FREDERICK E. DRAKE AND PAUL 0. FARNHAM', OF BOONTON, NEW JERSEY, AS-

SIGNORS TO RADIO FREQUENCY LABORATORIES, INC., 0F BOONTON, NEW JERSEY,

.A CORPORATION OF NEW JERSEY ELECTRICAL AMPLIFIER CIRCUIT Application filed May 4, 1928.

This invention relates to electrical amplifier circuits adapted for use in association with vacuum or electron tubes of high impedance, such as the so-called tetrode or double-grid tube. A tube of this type may be operated according to either of two methods, which are frequently designatedl as the shield-grid method and the space-charge method. In operation according to the former method the inner grid of the tube is employed as the control element while the outer grid, which in certain commercial constructions may almost completely surround and shield the anode, is given a positive bias or has applied to it a positive direct current potential by a,connection to the Bbattery or socket-power device, or by any other suitable means. In operation according to the latter method the outer grid of the tube, that is, the grid adjacent orv surrounding the anode, is employed as the control element while the inner grid of the tube has applied to it a positive potential.

ur invention is applicable to electrical amplifier circuits employing tetrode or double-grid tubes designed to operate according to either one or both of the foregoing v methods, but we have found it to be especially applicable to amplifier circuits employing four element tubes of the shield-grid type, i. e. to tubes designed to operate according to the first method described above. Our invention will therefore be hereafter described with reference to this application of the invention by way of specific example.

In electrical amplifier circuits designed to operate at radio frequencies, andyespecially in such circuits of the usual type in which the coupling system between adjacent tubes, or one 0r more of the resonantcircuits associated with a single tube, includes fixed inductances tuned by variable tuning capacities, the voltage amplification of the amplifier is ordinarily considerably higher at short wavelengths than at long wavelengths. Thus, in the case of the present broadcast range of wavelengths, say from approximately 200 to 550 meters, a single electrical amplifier stage employing a tetrode of the shield-grid type may give. a voltage gain or Serial No. 275,179.

put terminals of the amplifier and Eo is the voltage across the input terminals of the amplifier) is plotted as ordinates against'wavelength as abscissae, the resulting curve representing the amplification characteristic of the amplifier will, in general, have a relatively steep slope. That is, the amplification will be relatively high at short wavelengths and will fall off continuously and rapidly, becoming relatively low at long wmf'elengths. A typical amplification curve of this type for a tetrode amplifier is shown at A in Figure 5, as will be hereafter more fully explained.

. It is undesirable for an electrical amplifier to possess an amplification characteristic of this type, for a considerable number of reasons. For example, a radioreceiving set including such a radio frequency amplifier, if designed for proper amplification at the longer wavelength end of its range, will prove to be excessively sensitive toward the shorter wavelength end of its range. In the case of such' a radio frequency amplifier the high short-wavelength gain may tend to makel the whole' amplifier unstable, and may even cause it to break into oscillations at shorterwavelengths, due to stray capacitive and inductive couplings in the amplifier which 'couplings increase with frequency and are a maximum of the short-wavelength end of the range.

. In order to 'overcome the foregoing diiii- /culties, and others which are attendant upon an amplification characteristic ofthe usual type, it is desirable .for an electrical amplifier to have an amplification characteristic which remains of substantially the same order of magnitude over a pre-determined range of wavelengths, i. e. the amplification characteristic curve should preferably be substantially fiat or constant, and in many cases it is even desirable that the amplification or gain should be less at short wavelengths than at longer wavelengths in order to produce substantially equal stability of the amplifier throughout its Wavelength range.

lVe have found that the above described and other difhculties may be substantially overcome, and that the amplification characteristic of such an electrical amplifier may be materially modified in a desirable manner,

over Wide limits of Wavelength, by the use"of resistance of proper value or magnitude, arranged in proper manner, as will be hereafter described, in the plate or outputcircuit of the tube.

Referring now to the drawings, Figure 1 illustrates diagrammatically an electrical amplifier circuit including a single doublegrid tube, and arranged according to our invention. Figure 2 represents a cascaded or g5 multi-stage radio frequency amplifier embodying the circuit of'Figure 1 and arranged according to our invention. Figure 3 represents another form of electrical amplifier arranged according to our invention and employing a single tube of the double-grid type. Figure 4 represents still another electrical amplifier circuit to which our invention is applicable. Figure 5 illustrates certain typical amplification characteristic curves which will 3a be hereafter explained.

Referring now to Figure 1, VT is a vacuum or electron tube provided with an anode or plate, P, a cathode or filament, F, and two grid electrodes, Gr1 and G2. One of these ichag-,Qi electrodes, G1, constitutes the control elemen the tube, and the other grid electrode, Gf, Whic ay partially or completely surround the anode or plate, P, has applied to it a positive direct current potential by a 4r suitable connection to the Bbattery, B, or otherwise. The input terminals for the amplifier are 1 and 2, l being connected to the control element, G1, and 2 being connected to ground and to the cathode, F, thru grid biasing resistance Ro. A Cbattery may be alternatively employed for biasing the grid G, if desired, the terminal 2 being then connected to the cathode. The input voltage, E0, is the voltage across these input termia. nals. The cathode, F, may be heated either by direct or alternating current, or may be of the separate heater type, but is here illustrated as being operated from an Abattery, A. The terminal 2 may be grounded as shown HH at G, as described above. The plate or output circuit includes an inductive lmpedance, L2,

which is tuned by the tuning condenser C2. This inductance L2 is arranged so as to constitute an auto-transformer, the entire inal ductance, or any desired portion thereof, coristituting the effective secondary L2, of the transformer, across which the tuning condenser C2 is connected; and a portion of this inductance, L1, or the Whole inductance if desired, constituting the effective primary of the auto-transformer. The lower terminal of the auto-transformer may be connected to ground, The anode, P, is supplied with direct current potential from the Bbat tery, B, through radio frequency choke coil L5. C5 is a coupling` or blocking condenser of' low radio frequency impedance. It serves to keep the direct current potential of the anode from the windings of the transformer .Lf-L2. The plate `circuit of the tube, i. e. the primary of the output circuit of the amplifier, thus includes the inductance L1. It 'also includes resistance, here shown at R1, of a, proper value to produce the desired favorable modification of the amplification characteristic of the amplifier, as will be more fully eX- plained hereafter. The resistance R1 in this output circuit primary may be either lumped or distributed throughout the circuit as by inclusion in the self-resistance of coil L1, as Will be later more fully explained. "When it has the form of a separate resistance, R1, as shown in Figure 1, it may, of course, be connected on either side of the primary inductance L1, i. e. it may be connected between the cathode, F, and coil L1. It may also be either adjustable oi fixed, as is especially described hereafter. The output terminals of the amplifier are 3 and 4, and output voltage, E, is the voltage across these terminals.

Figure 2 represents a multi-stage radiofrequency amplifier according to our invention, as embodied in a radio receiving set. The antenna or other collecting device is connected through capacity Co and coil Lo to ground. The tuned input circuit Lo-Co includes the coil L0 and the variable tuning condenser C0. The amplifier is here illustrated as comprising two tubes, VT #l and VT #2, although it may of course include any desired number of tubes. The circuit for each of these tubes is arranged substantially as shown in Figure 1, the input terminalsof the first tube being the terminals 1 and 2 of the tuned circuit L0-C0, and the input terminals of the second tube beingtle terminals 1 and 2 oftuned circuit laf-C2. The output terminals of the radio frequency amplifier are 3 and 4, i. e. the terminals ofthe tuned circuit TX2-CQ, and may be connected to a detector or any other desired load. It should of course be understood that in such multi-stage arrangements it is possible to employ common A and B batteries for the several stages, and one suitable method of doing this is illustrated in Figure 2L The grids maybe biased by a common Cbattery or by the voltage drops across a biasing resistance Ro as shown, or in any other suitable manner. The batteries or biasing resistlllfl ances may be supplied with suitable by-pass condensers asis well understood in this art. Although A and B batteries and a biasing resistance are herein illustrated as means for supplying heating current to the cathodes, space current to the anodes and positive bias to the second grids, and negative bias to the control grids, respectively, it should be understood that any other suitable means of supplying these currents and potentials may be employed instead of the means herein illustrated. For example, socket-power .devices may be suitably employed, or alternating current or separately heated cathodes may be employed. In a multi-stage arrangement Lsuch as that shown in 'Figure 2 the several tuning condensers may be grounded as shown.

Figure 3 illustrates an alternative circuit arrangement for an electrical amplifier according to our invention. This circuit is substantially similar to the circuit rep-resented in Figure 1, except that in this case the autotransformer L1-L2 of Figure 1 is replaced by a transformer having separate primary and secondary windings L1 and L2 as shown in Figure 3. In this arrangement, it is of course possible to employ a separate resistance R1, which may be lumped or not, in the' plate circuit of the vacuum tube (i. e. in the output circuit primary), together with the primary L1, and connected on either side thereof, as in the arrangement of Figure 1. We prefer, however, in the arrangement of Figure, to introduce the resistance, B1, o f required magnitude, into the output circuit primary by Winding the primary coil L1 with wire of suitable resistivity. For example, we'may employ high resistance wire such as nichrome or other commercial resistance wire, for this purpose. Inthis arrangement the resistance R1 is accordingly constituted,

either wholly or in part, as desired, by the n self-resistance of the primary or plate coil,

' L1. Wemay, of course, combine a primary,

L1, of high self-resistance Vith an external or separate resistance,-R1, i'n order to bring the -total resistance of the plate circuit to the required value. i

Figure 4 represents an alternative arrange ment employing the invention dey cribed and claimed in the copending appli` ation, Ser. No. 275,177, filed May 4, 1928, of Frederick H. Brake. In this arrangement the autotransformers T1 and T2'in-clude a current winding, L2, which carries the main reso nantrurrent of the tuned circuits Lg-'Cg this current winding being composed of relatively large wire, and a twin potential winding L1, which is closely associated and coupled with the winding L2, or at least with al part thereof, and which servesas a means ne wire. This arrangement forms the subject matter of the above-mentioned copending application, Ser. No. 275,177, and is not claimed herein; but We have found that it may be adapted to the present invention by winding the fine coil, L1, with wire of such resistance as to fulfill the requirements herein described, or by connecting in series with the fine potential winding L1 a lumped resistance R1, of such a value as to fulfill these requirements.

It should, of course, be understood that any of the circuit arrangements illustrated herein, or variations thereof as indicated, are applicable not only to electrical amplifiers including a single stage, but to such amplifiers of the multi-stage type including any desired number of tubes or stages.

Figure 5 shows typical single-stage amplification characteristic curves A, B, C and D obtained by plotting 'the amplification or gain, E/Eo, as ordinates against wavelength as abscissae. Curve A represents a curve derived from experimental measurements on an electrical amplifier circuit including a tetrode arranged as shown in Figure 4, the fine winding L1 being composed, however, of copper wire of negligible resistance, and not therefore embodying our invention. In this circuit the following constants for the output transformer Ill-L2, were employed:

L1=55 turns No. 36 copper wire wound on 11/2 form.

L2=85 turns No. 28 copper wire wound on 1% form.

Curve B represents the amplification characteristic curve, as obtained experimentally, for a single stageelectrical amplifier circuit according to Figure 4, and embodying our invention. The output transformer and circuit had the' following constants:

L1 55 turns No. 36 nichrome wire Woundv ance of 575 ohms.

L2=85 turns No. 28 copper wire wound on 11/2 form.

It will thus be seen that curve B was obtained experimentally with an electrical amplifier circuit in which R1 was constituted by the self-resistance of the twin winding L1 of the auto-transformerLl-Lg of Figure 4. It will be noticed that the amplificationcurve of this amplifier has been substantially equalizedover the wavelength range from 20D-550 meters, in a highly desirable manner, by the use of resistance R1 as described. Moreover, this modification of the amplitication'curve has been accomplished by a reduction in the short Wavelength gain of the amplifier without substantial or serious decrease of its amplification at longer wavelengths.

Curve C is a curve obtained with a circuit similar to that of Figure 4, except that instead of winding the coil L1 withfine resistance wire, this coil Was composed of fine copon 11/2 form and having self-resistper wire and a separate lumped resistance R1 was connected in series therewith between the coil L1 and the anode. The constants of this circuit were as follows:

L1=55 turns #36 copper wire wound on lil/2 form.

L2=85 turns #28 copper wire wound on 11/2 form the winding Ll constituting a twin winding with a part of L2.

R1=230 ohms, lumped resistance, connected between L1 and the anode.

Curve D is a curve obtained with a circuit similar to that shown in Figure 1, the constants being as follows:

L1=L2=80 turns #3() copper wire on 11/5 form.

R1: 127 ohms lumped resistance connected between L1 and the anode.

It will be noted that in this particular instance the variable tap on L1 was at the top of the coil.

The foregoing circuits and constants are, of course, merely typical examples illustrative of our invention, which is not limited thereto.

The desired modification or correction of the amplification curve of an electrical amplifier according to our invention may be obtained by the use of a lumped resistance R1 in the plate or output circuit primary of the amplifier whether the resistance R1 takes the form of a separate resistance connected in series with the primary, L1, on either side thereof, or is distributed in the prima-ry circuit, as by including the resistance R1 in the self-inductance of the primary L1. Although either of these arrangements produces the desired result, in general, in order to obtain the same degree of desirable modification of the amplification curve by the two arrangements, somewhat different values or amounts of resistance in the plate circuit will be required. Moreover, the effect of a given magnitude or value of the resistance R1 depends somewhat on the other constants of the circuit. For example, referring to the arrangement shown in Figure 1, the larger is the portion of the coil Lg which is included as the effective primary, L1, of the auto-transformer of Ll-L2 for any given constant value of resistance R1, the greater is usually the effect of this resistance in decreasing the short wavelength gain of the amplifier without substantially affecting its long wavelength gain.

It is not possible to give any numerical value for the resistance R1 which is applicable to all circuits including our invention, since the value of this resistance must depend uponyand be adapted to the constants of the tube and of the circuit, including the value of the inductance L1 as explained above. Our invention extends to all cases in which the resistance of the plate circuit (i. e. output circuit primary) exceeds the normal self-resistance of this circuit when L1 is wound with low resistance wire such as copper, by any amount sufficient to produce a favorable modification of the amplification characteristic of the amplifier. However, we may state that, in general, for anyparticular circuit and tube, we prefer to employ, according to our invention, a resistance R1 of such value that the amplification of the electrical amplifier circuit remains of substantially the same order of magnitude over a predetermined wavelength range over which the amplifier is designed to operate. Within these limits the amplification may remain substantially constant, but it is also within the definite contemplation of our invention that the resistance R1 may have such a value that the amplification is actually lower at short wavelengths than at long wavelengths, in order to produce increased stability of the amplifier, or that the amplification is somewhat higher at short or at intermediate wavelengths than it is at long wavelengths in order to produce increased sensitivity at these wavelengths if desired. By amplification, or an amplification characteristic curve, which remains of substantially the same order of magnitude over a predetermined wavelength range, or which remains substantially constant over such a range, we do not mean an amplification or amplification curve which is actually fiat or constant, but refer to amplification, or an amplification characteristic curve, such as that represented by curves B, C and D of Figure 5.

Circuits according to the present invention,` l1n which resistance of required or predetermined value is included in the plate or primary circuit of the electrical amplifier output, as herein described, in order to effect a desirable modification of the amplification characteristic curve of the amplifier, should, of course, be distinguished from balanced circuits of the type described and claimed in the copending application of Frederick H. Drake, Ser. No. 275,178, filed May 4, 1928, in which resistance is included in the primary of a balanced circuit in order to produce a controlled resistance balance or unbalance of desired character while permitting a reactive balance or desired approach thereto. The presentinvention does not relate primarily to such balanced circuits of the type having a controlled resistance balance or unbalance,

.but relates more especially to circuits, such as those described herein, in which the effect of the resistance de ends upon complicated mathematical considerations which we have not as yet completely worked out, but is probably, in general, according to our best present knowledge, a combination selective coupling and loss effect of a character depending upon the circuit and tube constants in the manner described above.

According to our invention the resistance,

R1, of the value described above, may ofv course be variable as indicated in Figures l meagre and 4, either at the will of the operator or by a manufacturers or laboratory adjustment of the amplifier to produce the desired modification of the amplification characteristic thereof, or may be iiXed at a predetermined value which is arrived at either emperically or by calculation in the design of the amplifier, as is shown in Figures 2 and 3.

By a transformer as the Word is used in the speciiication and claims herein, We include both an auto-transformer of the types shown in Figures l, 2 and 4 and a transformer having separate primary and secondary windings as shown in Figure 3. By the Word resistance as used herein We include both a variable or adjustable resistance and a iiXed resistance, unless otherwise specifically designated,-

The foregoing details of constructions and constants are merely specific examples according to our invention, which is not limited theretoc Moreover, its application is not limited to electrical circuits of the type illustrated, these being chosen merely as examples of the many .applications of our invention.

`Many variations and modifications of the specific circuit arrangements and transformer constructions described herein, fall Wi thin our invention. A

Our invention is not limited to the particular circuit arrangements or circuit constants given herein, since these are merely illustrative of the invention, but extends to suitable modiiications and variations thereof embodying the teachings of the foregoing specification.

l. An electrical amplifier circuit comprising, in combination, an electron tube including an anode, a cathode, a control grid, and a second grid; an input circuit associated with said control grid; means for imparting a positive potential to said second grid; and a tuned output circuit associated with said anode and cathode, said output circuit primary including inductance and resistance of such a value in excess of the normal inherent self-resistance of said circuit, said resistance being of such magnitude that its presence in the output circuit eil'ects a substantial reduction in the amplification of signals at the lower range of the Wave-length band to which the output circuit may be tuned, While causing approximately no change in the amplification at the upper range.

2. An electrical ampliiier circuit comprising, in combination, an electron tube including an anode, a cathode, a control grid, and a second grid; an input circuit associated With said control grid; means i'or imparting a positive potential to said .si-cond grid; and a tuned output` circuit associated with said anode and cathode, said output circuit includ-' ing inductance and resistance of a value such that the amplification of said amplifier cir-v cuit remains of substantially the same order of magnitude over a predeterminedrange of wavelengths,

3. An electrical amplifier circuit comprising, in combination, an electron tube including an anode, a cathode, a control grid, and a second grid; an input circuit associated With said control grid; means for imparting a positive potential to said second grid; and a tuned toutput circuit associated with said anode and cathode, said output circuit including inductance and resistance of a value such that the amplification of said amplier circuit remains substantially constant over a predetermined range of Wavelength.

et. An electrical ampliiier circuit comprising, in combination, an electron tube including an anode, a cathode, a control grid; and a shield-grid; an input circuit connected between said control grid and said cathode; means for imparting a positive potential to said shield-grid; and an output circuit associated With said anode and said cathode and including a tuned radio frequency transformer having primary inductance connected between said anode and said cathode to form a plate circuit for said tube, said plate circuit having resistance of such value that the ampli cation of said amplier circuit remains of substantially the saine order of magnitude over a predetermined range of Wavelength.

5. An electrical amplifier circuit comprising, in combination, an electron tube including an anode, a cathode, a control grid, and a shield-grid; an input circuit connected 'between said control grid and said cathode; means for imparting a positive potential to said shield-grid; and a tuned output circuit associated With said anode and said cathode and including a tuned radio frequency transformer having primary inductance connected between said anode and said cathode to form a plate circuit for said tube, said plate circuit having resistance of such value that the amplification of said amplifier circuit remains substantially constant over a predetermined range of Wavelength.

6. An electrical amplifier system, `comprising, in combination, a tube having at least an anode, a cathode, a shield-grid, and a control element; anda transformer having a current Winding consisting of relatively large wire.

and a potential winding consisting of rela-- tively small Wire closely associated and coupled With at least a part o'f said current Winding and forming a twin Winding therewith, said potential Winding being associated with a source of direct current potential and with said anode and cathode of said tube in the plate circuit thereof and having resistance of such a value in excess of the normal inherent self-resistance of said circuit as to have approximately no effect upon the amplification of signals at the upper range of the Wavelength band towhich said output circuit may be tuned and to effect a substantial reduction in the amplification of signals at the lower range thereof.

7. A multistage electrical amplifier comprising, in combination, a tube having an anode, a cathode, a control grid, and a second grid; a transformer having a current winding consisting of relatively large wire and a potential winding consisting of relatively small resistance wire closely associated` and coupled with at least a part of said current winding and forming a twin winding therewith; a condenser shunted across one of said transformer windings and adjustable to tune said amplifier over a band of wavelengths; a connection from said cathode to said anode through a source of direct current potential and through said potential Winding and forming the plate circuit of said tube; a second tube having an anode, a cathode, a control grid, and a second grid; connections whereby said current winding of said transformer is associated with said control grid and cathode of said tube; and means for ap-Y plying a bias to said second grids; said resistance wire winding in said plate circuit of said first tube having such a value in excess of the normal inherent self-resistance of said circuit as to produce a substantial reduction in the amplification of signals falling Within a limited predetermined range of less extent than the band of wavelengths to which said amplifier may be tuned.

S. A multi-stage electrical amplifier cornprising, in combination;a tube having an anode, a cathode, a control grid, and a second grid; a tuned transformer having a current winding and a potential winding consisting of relatively small resistance wire closely associated and coupled with at least a part of said current winding and forming a twin winding therewith; a connection from said cathode to said anode through a source of direct current potential and through said potential winding and forming the plate circuit of said tube; a secondtuhe having an anode, a cathode, a control grid, and a second grid; connections whereby said current winding of said transformer is associated with said control grid and cathode of said second tube; and means for applying a bias to said second grids; said resistance wire Winding in said plate circuit of said first tube having such a value that the amplification of said amplifier circuit remains of substantiall the same order of magnitude over a pre etermined range of' wavelengths.

9. A multi-stage electrical amplifier comprising, in combination, a tube having an anode, a cathode, a control grid, and a second grid; a tuned transformer having a current winding consisting of relatively largewire and a potential winding consisting of rela tively small resistance wire closely associated and coupled with at least a part of said current Winding and forming a twin winding therewith; a connection from said cathode to said anode through a source of direct current a,

potential and through said potential winding and forming the plate circuit of said tube; a second tube having an anode, a cathode, a control grid, and a second grid; connections whereby said current winding of said transformer is associated with said control grid and cathode of said tube; and means for applying a bias to said second grids; said resistance wire winding in said plate circuit of said first tube having such a value that the amplification of said amplifier circuit remains substantially constant over a predetermined range of Wavelength.

In testimony whereof, we affix our signatures.

FREDERICK H. DRAKE. PAUL O. FARNHAM. 

